U.S. Pat. No. 4,232,686 which issued to Kammlade Jr. in 1980 and U.S. Pat. No. 4,319,583 which issued to Ingle in 1982 both disclose remote parturition monitoring systems for use with livestock. The transmitter disclosed in the Kammlade reference consists of a rupturable assembly which is mounted externally across the vaginal orifice of the animal. The transmitter disclosed in the Ingle reference consists of a magnet positioned on one side of an animal's vulva and a switch mechanism positioned on the other side of an animal's vulva. When the magnet is moved out of the field sensing area of the switch mechanism during parturition, the switch mechanism activates a transmitter.
U.S. Pat. No. 4,936,316 which issued to Jewett in 1990 describes problems of false triggering which have been encountered in prior art transmitters, such as those taught by Kammlade and Ingle. With the Kammlade reference problems of false triggering are described as arising if great case is not taken to ensure correct installation. With the Ingle reference, false triggering occurs from any movements of the animal which displace the magnet outside of a preset range. False triggering can occur when the animals rub against fixed objects. Apparently, such rubbing is common as the animals attempt to relieve the sensory perceptions associated with the presence of the device. The solution to false triggering, as taught by Jewett, consists of placing a magnet in a housing positioned on one side of the animals vulva and attaching a thong between the magnet and the other side of the animals vulva. Upon parturition occurring, the magnet is pulled from the housing by means of the thong, thereby triggering a switch that activates the transmitter.